The present invention relates to the epitaxial growth of semiconductor materials in a manner which produces monocrystal material with improved characteristics, and to the structures based on such material. In particular, the invention relates to a method of fabricating semiconductor materials with the bandgap width exceeding 1.8 eV on a porous layer of a monocrystal semiconductor material.
Semiconductor materials with bandgap width exceeding 1.8 eV have, long been considered as materials of choice for high temperature and high power devices, due to their high thermal conductivity and robust mechanical and chemical properties. They are also required in green, blue, violet and UV-bandwidth optoelectronics, because their bandgap values correspond to these areas of the spectrum. Of those materials, most commonly used today are Silicon Carbide, Gallium Nitride, Aluminum Nitride, and related materials, such as BN, AlGaN, etc. Devices based on these materials are currently on the industrial market and proved to be invaluable in many areas, starting from consumer electronics and up to avionics and space based power systems. However, the production of commercial devices based on abovementioned materials is still fighting with serious problems related to the quality of the original material. Despite the progress in bulk crystal growth techniques and the development of modern advanced technologies, the actual quality of wide bandgap epitaxial materials still restricts possible high-power and high-temperature device applications of these materials. Though it is possible today to grow wafers of these materials as large as 5 inches in diameter, the whole area of these wafers cannot be used for device structures because of a number of structural and point defects present in the wafer. That makes small device structure based on pieces cut from the wafer more expensive and prevents the fabrication of very-large-area devices needed for extremely high current densities required in to-day""s power devices.
So, it is believed that the key element in the development of wide-bandgap semiconductor electronics is a proper substrate. The main limiting factors currently precluding the wider use of silicon carbide epitaxial layers is relatively high defect density in silicon carbide substrates, while for group III nitride materials the problem is in a substrate as such. In the case of SiC, defects from the substrate penetrate inside the bulk SiC crystals and epitaxial layers grown on the said substrate. These defects are limiting material characteristics and device performance.
There is no material which could be used as a native substrate for epitaxial growth of group III nitride materials, and poor lattice match and difference in thermal expansion coefficients with foreign substrates being currently used for epitaxial growth of group III nitride layers make these layers quite strained, with average value of biaxial stress ranging up to 1 GPa. This stress affects both structural and electric properties of the layers and devices built on such layers cannot take full advantage of intrinsic properties of the materials. Some researches have attempted to solve the problem by growing epitaxial layers on various buffer layers, such as thin AlGaN layer on SiC wafer for the growth of GaN epitaxial layer, on which the device structure would be based. This attitude helps, yet defects such as so called nanopipes, inclusions, dislocations and stacking faults that are present in the original substrate wafer still propagate in epitaxial structure that is grown on that substrate and eventually lead to the device degradation at particular power and/or temperature levels.
Therefore, it is an object of the present invention to provide monocrystal epitaxial layers of wide bandgap semiconductors with improved structural, electrical and optical characteristics and resulted structures with improved performance.
The invention meets this object with a method of growing semiconductor materials on a porous monocrystal layer being made of material with the bandgap width exceeding 1.8 eV. This porous layer is produced during the first step of fabricating the material, through electrolytic treatment of the wafer at direct current under or without UV-illumination.
In another aspect, the invention comprises a method of fabricating group III nitride material using the abovementioned two-steps fabricating process.
In yet another aspect, the invention comprises a method of fabricating Silicon Carbide monocrystal layer using the abovementioned two-steps fabricating process.
In yet another aspect, the invention comprises a semiconductor device comprising at least one layer of porous group III material, where this material has an average pore spacing of less than 1 micron.